Electrode graphite is usually synthetically produced graphite, which in addition to the well-known use in electric arc melting processes in steel mills is used in fields such as tool and mould making for EDM (spark erosion). Therein, the trend is towards ever more filigree structures of the workpieces to be manufactured with the graphite electrodes, thus leading to an increased demand for precision tools for processing electrode graphite.
The milling tools employed for machining electrode graphite therefore have high requirements in terms of both accuracy and fatigue strength, because graphite is a highly abrasive material that requires high cutting speeds during machining and due to the resulting abrasive dust grains causes rapid wear and thus rapid loss of the milling tool.
Cutting tools created especially for graphite machining therefore have corresponding tool geometries which are adjusted to a hardness of up to 90 Shore and high abrasion of the material due to the carbon grains produced during machining, while at the same time having tight manufacturing tolerances due to the fine graininess of the material (up to 0.5 μm is possible).
In general, therefore, the production of a graphite electrode takes place in two to three steps, wherein first rough-machining takes place in which as much material as possible is removed in as short a time as possible. It is then smoothed or finely processed, often with a pre-smoothing operation and a finishing operation, in which the exact final geometry of the electrode is then milled out of the electrode graphite blank.
European patent publication EP 2 540 427 B1 shows, for example, a cutter for electrode graphite with a cutting head in ball head geometry, as well as JP 08141816 A. This results in a high dimensional accuracy in the fine machining, even with complicated workpiece geometries. Cutters for electrode graphite with other tool geometries with a plurality of cutting plates on the front side or on the circumference of the milling cutting edge, on the other hand, are more suitable for the quickest possible rough machining, see DE 102 47 715 A1 for example with respect to the cutting plates on the end face of the milling cutter, and DE 10 2005 044 015 B4 with respect to tooth-like mounted cutting plates on the circumference of the milling cutter.
From the machining of other, less abrasive and brittle materials such as CFRP which are also difficult to machine, the concept is already known to provide on a single tool both roughing and smoothing cutting edges. For example, DE 10 2012 019 804 A1 shows a face milling cutter for machining fiber-reinforced materials such as CFRP, which includes pre-machining lands with teeth for roughing and post-machining lands provided with circumferential smoothing cutting edges provided on exterior grooves for smoothing or re-reaming. As a result, it is possible to rough-machine or coarsely work and to finish or finely machine with a single milling tool. During finishing, the thread ends of the fiber-reinforced plastic protruding from the workpiece after roughing are separated. One operation is thus saved. The principle is to distribute tasks on differently shaped cutting edges during loads or tasks occurring during machining.
This principle has also been adopted in a milling cutter for machining graphite, which is shown in U.S. Pat. No. 6,164,876. It shows a milling cutter with the ball head geometry necessary for the free-form machining on final accuracy of graphite workpieces, which milling cutter has four lands, two of which are designed as rough-machining lands with chip breaker grooves and two as fine-machining lands with cutting edges extending along the lands. While pre-fragmentation of the material takes place on the rough-machining lands with the local grooves introduced transversely to the tool axis, the cutting edges twisted together with the fine-machining lands around the tool axis in a right-twisted manner are used for post-reaming.